The long-range goal of this project is to understand how descending neural systems modify information processing in the cochlea and cochlear nucleus. Existence of descending innervation to hair-cell systems in most vertebrate species examined is strong presumptive evidence for the importance of descending systems. There is evidence that the function of descending systems may be: 1) to reduce the masking effects of continuous backgrounds of noise, 2) to protect the inner ear from over- exposures by intense sound, 3) to expand the limited dynamic range of auditory-nerve fibers, and 4) to enable selective attention to certain signals. These functions have direct health relevance; their interruption would severely damage important processes like speech discrimination and render the inner ear more vulnerable to damage. This project will concentrate on one particular one particular descending, the medial olivocochlear (MOC) neurons, in guinea pigs and in mice. MOC neurons respond to sound and form a reflex that sends information to the outer hair cells of the cochlea. Although we know that MOC neurons reduce the effects of noise masking and protect from over-exposures, we know little of the mechanisms by which the MOC reflex acts. Our experiments will examine electrophysiological responses of single MOC neurons to noise and to over-exposures that cause threshold shifts. We will also examine the mappings of single MOC neurons labeled by injections of neural tracers. These experiments will answer questions like, "What mechanisms enable the MOC reflex to reduce the effects of noise masking and to protect the inner ear from over-exposures?", and "What is the tonotopic mapping of the MOC neurons to the outer hair cells?" We will also study a group of small auditory neurons that have been difficult to study with electrophysiological methods These type II neurons of the spiral ganglion are related to the MOC neurons because they provide the afferent innervation of the outer hair cells, and because they give off branches in the same region of the cochlear nucleus in the brain. However, the role of type II neurons in hearing is not known. Studying the peripheral and central targets and their relationship to MOC neurons are likely to reveal properties important to their function. This knowledge would greatly increase our understanding of both the descending and the ascending auditory pathways, and elucidate their roles in the hearing process.